This document provides an overview of synapses, including their definition, structure, function, types of transmission (electrical vs. chemical), neurotransmitters, and various properties like synaptic delay, fatigue, summation, and more. It discusses excitatory and inhibitory neurotransmitters and how convergence and divergence allow signals to be dispersed or combined. Clinical implications are that problems with synaptic transmission can cause diseases like Parkinson's and Alzheimer's.

1. Preshantha Govender
Roll #24

2. 1. Definition
2. Structure
3. Function
4. Types
5. Synaptic Transmission
A. Electrical
B. Chemical
6. Transmission of Neurotransmitters
7. Excitatory & Inhibitory Neurotransmitters
8. Kiss & Run Docking
9. Properties
A. One-way Conduction
B. Synaptic Delay
C. Fatigue
D. Summation
E. Facilitation
F. Excitatory & Inhibitory
G. Occlusion Phenomenon
H. Convergence & Divergence
10. Clinical
11. References

3. • The junction between two neurons is called a synapse.
• It is a specialized junction where transmission of
information takes place between a nerve fibre and
another nerve, muscle or gland cell.
• It is not the anatomical continuation. But, it is only a
physiological continuity between two nerve cells.

4. The synapse consists of:
1. A presynaptic ending that contains neurotransmitters,
mitochondria and other cell organelles.
2. A postsynaptic ending that contains receptor sites for
neurotransmitters.
3. A synaptic cleft or space between the presynaptic and
postsynaptic endings. It is about 20nm wide.

6. • The main function of the synapse is to transmit the
impulses, i.e. action potential from one neuron to another.
• They allow integration, e.g. an impulse travelling down a
neuron may reach a synapse which has several post
synaptic neurons, all going to different locations. The
impulse can thus be dispersed. This can also work in
reverse, where several impulses can converge at a
synapse

7. 1. Synapse with another neuron
It is the junction between two nerve cells. They are of 3
types; axodendritic, axosomatic & axoaxonic
2. Neuromuscular
It is the synapse pf a motor neuron and a muscle
3. Neuroglandular
It is the synapse of a neuron and a endo/exocrine gland.

10. • It is the process which nerve cells communicate among
themselves or with muscles and glands.
• The synapse is the anatomic site where this
communication occurs.
• It can be of 2 types:
A. Electrical transmission
B. Chemical transmission

11. In these synapses the membranes of the two cells actually
touch, and they share proteins. This allows the action
potential to pass directly from one membrane to the next.
They are very fast, but are quite rare, found only in the
heart and the eye.
In a chemical synapse, electrical activity in the presynaptic
neuron is converted into the release of a chemical called a
neurotransmitter that binds to receptors located in the
plasma membrane of the postsynaptic cell.

13. I. At the end of the pre-synaptic neuron there are voltage-
gated calcium channels. When an action potential
reaches the synapse these channels open, causing
calcium ions to flow into the cell.
II. These calcium ions cause the synaptic vesicles to fuse
with the cell membrane, releasing their contents (the
neurotransmitter chemicals) by exocytosis.
III. The neurotransmitters diffuse across the synaptic cleft.
IV. The neurotransmitter binds to the neuroreceptors in the
post-synaptic membrane, causing the channels to
open.

15. • Inhibitory neurotransmitters produce a depolarization of
the postsynaptic membrane called the inhibitory
postsynaptic potential (IPSP). They reduce chances of a
new impulse.
I. Serotonin
II. GABA (γ aminobutyric acid)
III. Glycine
• Excitatory neurotransmitters can cause the next cell to
initiate a new impulse.
I. Acetylcholine
II. Norepinephrine
III. Dopamine

16. • Kiss-and-run docking is a type of synaptic vesicle release
where the vesicle opens and closes transiently.
• In this form of exocytosis, the vesicle docks and
transiently fuses at the presynaptic membrane and
releases its neurotransmitters across the synapse, after
which the vesicle can then be reused.
• Kiss-and-run differs from full fusion, where the vesicle
collapses fully into the plasma membrane.

17. (Bell-Magendie Law)
According to Bell-Magendie law, the impulses are
transmitted only in one direction in synapse, i.e. from
presynaptic neuron to postsynaptic neuron.

18. • During the transmission of impulses via the synapse,
there is a short delay in the transmission. It is due to the
time taken for:
i. Release of neurotransmitter
ii. Movement of the neurotransmitter from the axon
terminal to the postsynaptic membrane
iii. Action of the neurotransmitter to open the ionic
channels in the postsynaptic membrane

19. • During continuous muscular activity, the synapse forms
the seat of fatigue along with the Betz cells.
• The fatigue at the synapse is due to the depletion of
neurotransmitter substance, acetylcholine.
• Depletion of acetylcholine occurs by two factors:
I. Soon after the action, acetylcholine is destroyed by
acetylcholinesterase
II. Due to continuous action, new acetylcholine is not
synthesized.

20. • Synapses require the release of sufficient transmitter into
the cleft in order for enough of the transmitter to bind to
the postsynaptic receptors and the impulse to be
generated in the postsynaptic neuron.
• Summation is of 2 types:
I. Spatial summation occurs when excitatory potentials
from many different presynaptic neurons cause the
postsynaptic neuron to reach its threshold and fire.
II. Temporal summation occurs when a single
presynaptic neuron fires many times in succession,
causing the postsynaptic neuron to reach its threshold
and fire.

21. • When a pre-synaptic axon is stimulated with several
consecutive individual stimuli, each stimulus may evoke a
larger post-synaptic potential than that evoked by the
previous stimulus.
• Consequently, another excitatory signal entering the
neuron from some other source can excite the neuron
very easily.

22. • At an excitatory synapse, ionic currents flowing through
the ion channels cause a net depolarization of the
postsynaptic cell.
• At an inhibitory synapse, ionic currents flowing through
the ion channels cause a net hyperpolarization of the
postsynaptic cell.

23. • Convergence refers to the phenomenon of termination of
signals from many sources (i.e. many pre-synaptic
neurons on a single post-synaptic neuron).
• Divergence refers to one pre-synaptic neuron terminating
on many post-synaptic neurons. (i.e. single impulse is
converted into a number of impulses going to a number
of post-synaptic neurons.)

24. • The response to stimulation of 2 pre-synaptic neurons is
more than the sum total of the response obtained when
they are stimulated separately.
• This happens because some post-synaptic neurons are
common to both the pre-synaptic neurons.
• Thus occlusion is due to overlapping of afferent fibres
their central distribution.

25. A problem with communication between nerves at
synapses is often the basis for disease, like the following:
• Parkinson’s Disease
• Alzheimer's Disease
• Depression
• Anxiety
• Schizophrenia

26. Websites:
• www.wikipedia.org
• www.slideshare.net
• www.authorstream.com
Textbooks:
• Textbook of Medical Physiology, Guyton & Hall